Process of coating steel sheets with tin



' citizen of the UNITED STATES PATENT DFFICE.

SAMUEL PEACOCK, 0F WHEELING, WEST VIRGINIA, ASSIGNOR, BY MESNE ASSIGN-MENTS, TO WHEELING STEEL A CORPORATION OI WEST VIRGINIA.

& IRON COMPANY, or wnnnmn'o, wrsr VIRGINIA,

PROCESS OF COATING STEEL SHEETS WITH TIN.

No Drawing.

To all whom it may concern e it known that I, SAMUEL PEAcooK, a UnitedStates, residing at Wheeling, in the county of Ohio and State of WestVirginia, have invented certain new and useful Improvements in Processesof Coating Steel Sheets with Tin; and I do hereby declare the followingto be a full, clear, and exact description of the invention, such aswill enable others skilled in the art to which it appertains to make anduse the same.

This invention relates to a process of coating thin steel sheets withtin, and has for its object to improve the procedures heretoforeproposed.

"With these and other objects in view, the invention consists in thenovel steps and combinations of steps constituting the process, all aswill be more fully hereinafter disclosed and particularly pointed out inthe claims.

In order that this invention may be the more clearly understood, it issaid: In the present procedure of coating one metal with another, such,for example, as coating thin sheets of mild steel with tin, to form thetin plate of commerce, it is usual to free the steel sheets of oxide,grease, etc., and to then immerse them in a molten bath containing tin,at atmospheric pressure.

The differences in contraction between the steel plates and the tincoatings on cooling, although very small, are, yet, sufiicient to formfissures or cracks in the very thin tin coatings now employed.

These fissures are usually too small to be readily detected bymagnifying glasses, but may be readily detected by well known chemicaltests, and they offer a sufiicient exposure of the steel through thetincoating to start an electrolytic action when immersed in an electrolyticsolution, such, for example, as is afforded by many food products, suchas fruits and vegetables, when put up in tin cans.

It therefore follows that .for many industrial uses the present tinplates have a short life due to the inevitable corrosion that followstheir use, and the very serious, well known objections that saidcorrosion entails in producing salts that may be injurious to health.

It has been attempted to avoid these objec- Specification of LettersPatent.

. pheric pressure,

tlon is increased.

latented Nov. 8, 1921.

Application flied November 9, 1920. Serial No. 422,813.

tions by depositing the tin coating electrolytlcally on the steel plate,but such coatings invariably have a crystalline structure, and

are found in practice to be as objectionable provided from a moltenbath, and therefore no progress toward a solution of the difli culty ismade.-

I have discovered, on the other hand, that if instead of immersing thethin steel sheets in a coating bath of molten tin, under substantiallyatmospheric pressure, I substitute a solution of a tin salt, such asstannous chlorid SnCI and if I increase the pressure by means of heat tofrom say 5 to 15 atmospheres or higher, the advantages described beloware attained. That is to say, I have discovered it I maintain a lowionic concentration in the tin salt solution, it will deposit out thetin in a colloidal state, insteal of in a crystalline state, and thesteel will be covered completely and without fissures with said tin.

On the other hand, a low ionic concentration means a slow rate ofcoating. But I have observed that by heating the coating solution whilemaintaining a super-atmosthereon, the rate of deposi- This is especiallytrue, if the solution is agitated and the pressure is maintained at apoint sutiicient to prevent boiling of the coating solution. This ma) bereadily accomplished by the use of an autoclave.

I have further observed that the degree of ionic concentration governsthe colloidal nature of the metal coating, or the lower the ionicconcentration, the more colloidal will be the deposited tin. But caremust be taken to not employ such a low ionic concentration as willrender the rate of coating so slow as to be commercially inoperative.

As a practical operation in carrying out this invention, I may use, say,5 parts by weight of the stannous chlorid to 100 parts by weight of thediluent to be used, such as an ammonium alum or an alkali bi-tartrate.

I may next make up a 3% solution of said chlorid and alum or bitartratewith water, This said solution together with the sheets to be coated areplaced in an autoclave under a steam pressure of say between 5 and 15atmospheres, or preferably of about 10 atmospheres, when the tin will befound to be coated on the steel sheets in a firmly adhering non-porous,non-fissured condition, much superior to the coatings of the priorprocesses.

The plates are next removed from the autoclave, and dried, when they areready for use in making tin cans, or other articles.

The function of the ammonium alum or the potassium or sodium bitartrateis to lower or restrain the natural ionic concentration of the stannouschlorid. But, of course, other restrainers may be employed. The functionof the heat is undoubtedly to increase the kinetic energy of the tinions. Of course, the plates or sheets should be in a clean condition, orfree from scale, grease, etc., before being immersed in the solution.The latter should not have a concentration as high as 20% of thestannous chlorid, and from 3% to 5% is preferable.

It is obvious that those skilled in the art may vary the details of theprocedure without departing from the spirit of the invention, andtherefore, I do' not wish to be limited to the above disclosure exceptas may be required by the claims.

W'hat I claim is:

1. The process of forming a coherent coating of tin on thin steel sheetswhich consists in preparing a solution of stannous chlorid mixed with analum having an alkaline reaction adapted to restrain the ionicconcentration of said stannous chlorid; im-

mersing said sheets in a clean condition in said solution; andsubjecting the latter to a steam ressure exceeding two atmospheres,substantially as described.

2. The process of forming a coherent coating of tin on thin steel sheetswhich consists in preparing a solution of stannous chlorid mixed with analkali alum salt adapted to restrain the ionic concentration of saidstannous chlorid; immersing said sheets in a clean condition in saidsolution; and subjecting the latter to a steam pressure exceeding fiveatmospheres, substantially as described.

3. The process of forming a coherent coating of tin on thin steel sheetswhich consists in preparing a solution of stannous chlorid mixed with anammonium alum adapted to restrain the ionic concentration of saidstannous chlorid; immersing said sheets in a clean condition in saidsolution, and subjecting the latter to a steam pressure exceeding fiveatmospheres, substantially as described.

4. The process of coating thin steel sheets with tin which consists inpreparing a solution containing stannous chlorid, and an ammonium alum;immersing said sheets in a clean condition in said solution; andsubjecting said solution to a steam pressure exceeding five atmospheres,substantially as described.

5. The process of coating thin steel sheets with tin which consists inpreparing a solution containing less than 20% of stannous chlorid andammonium alum; immersing said sheets in a clean condition in saidsolution; and subjecting said solution to a steam pressure exceedingfive atmospheres, substantially as described.

Intestimony whereof I allix my signature.

SAMUEL PEACOCK.

